If either the Dutch or the 30 Years' War go better for Spain, Denmark mifght not look so outlandish a target. A naval invasion from Belgian ports is not at all far-fetched if the Dutch navy does (or can) not interfere, and Spanish troops would not meet marked resistance between the Lower Rhine and the Elbe if they really meant it.
Of courser, 17th century warfare kinda favoured the defender, but I can't see any greater implausibility in it that, say, an attempt to land troops in England or a Durtch involvement in naval warfare in the Mediterranean.